The relationship between sperm motility and fertilizing ability will be investigated under in vitro conditions, using the hamster as a model system. Chemically defined sperm motility-stimulating factors will be used to maintain small numbers of spermatozoa in a defined culture medium. The ability of hamster epididymal spermatozoa to penetrate the egg envelopes and gain access to the egg cytoplasm will be correlated with the motility of the spermatozoa, which typically show qualitative motility changes at the time of acquiring fertilizing ability. The possible functional role of the motility-stimulating substances in species other than the hamster (bull, rhesus monkey) will be studied. Physiological (fertilizing ability) and biochemical parameters will be evaluated.